1. Technical Field
The present disclosure relates to a non-woven filtration material and an air cleaner element using a non-woven filtration material.
2. Related Art
A non-woven filtration material is used for air filtration as a filtration material for air cleaners applied for, for example, an internal combustion engine and a fuel cell, which are used for an automobile and the like. In such an application, there is a demand for effectively removing fine dust from a large amount of air. In addition, the non-woven filtration material is required to have a high cleaning efficiency, a long life with which dust can be removed for a long period without causing clogging, and compactness with which a sufficient filtration performance can be exerted within a limited space.
Particularly in recent years, the air filters are required to improve the performance of removing particulate dust such as carbon dust. If particulate dust, such as carbon dust, cannot be fully removed, the dust is accumulated on a detection device, such as a flow sensor, which is disposed at the downstream side of the filtration material (filter element). This may result in insufficient accuracy and stability of the output of the detection device.
In the structure of a multilayer filter element of a typically-known non-woven filtration material, particulate dust, such as carbon dust, is often captured by a layer containing fibers having a small diameter and having a highest bulk density. Accordingly, when the carbon dust is captured, the pressure loss of the filtration material is likely to increase and the life of the filtration material is shortened. In particular, when the particulate dust capture efficiency (cleaning efficiency) is increased by, for example, increasing the bulk density of a dense layer, clogging of the filtration material is more likely to occur. In other words, in the filtration of particulate dust using the non-woven filtration material, there is a trade-off relationship between the cleaning efficiency and the life of the filtration material.
For example, JP-A-2005-349389 discloses a non-woven filtration material including a plurality of non-woven fabric layers which is not impregnated with oil. This non-woven filtration material is formed in such a manner that the porosity of the non-woven fabric layer disposed at the upstream side of the non-woven filtration material is smaller than the porosity of the non-woven fabric layer disposed at the downstream side of the non-woven filtration material. JP-A-2005-349389 also discloses that the porosity of the non-woven fabric layer disposed at the downstream side of the non-woven filtration material is in a range from 85% to 92%; the average fineness of the fibers contained in the non-woven fabric layer disposed at the downstream side of the non-woven filtration material is 3 decitex or less; and the average fineness of the fibers contained in the non-woven fabric layer disposed at the upstream side of the non-woven filtration material is 3 decitex or more. According to the non-woven filtration material, dust permeation due to intake pulsation is prevented. Accordingly, an advantageous effect that the carbon dust cleaning efficiency and the carbon dust retaining amount can be favorably maintained is obtained.